THE NUMBERS, EXPLAINED
How to use this calculator.
ISO weeks begin Monday; week 1 contains January 4. The ISO week-year can differ from the calendar year.
Start with your inputs
Enter date. Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Date | 2026-01-01 |
A worked example
With the example inputs above, iso week is 2026-W01. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| ISO week | 2026-W01 |
How this is calculated
ISO week
ISOweek(a)
Symbols and units
- a
- Date
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Calculation steps and notation
Use Monday-based weeks. The week-year is the year containing that week’s Thursday; week 1 contains 4 January. The displayed week-year can differ from the calendar year near New Year.
Assumptions and limits
ISO weeks begin Monday; week 1 contains January 4. The ISO week-year can differ from the calendar year.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
ISO week. The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: ISO weeks begin Monday; week 1 contains January 4. The ISO week-year can differ from the calendar year.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
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